The present invention relates to spherical rotary cutting tools (also called spherical cutting tools) such as ball end mills, tapered ball end mills and the like, in which cutting edges are formed on a spherical surface.
Particularly, the present invention relates to the use of cutting edges with a constant helix angle on the spherical surface. As a natural consequence, it improves the cutting performance, the chip disposal and the surface roughness of the cut material. Also, the cutting edges can be formed with an eccentric relief on the land, provided the helix angle is constant; the tooth formed by the eccentric relief is reinforced with a smaller relief angle and a wider land width. Due to the reinforcement, the tool can be operated at a higher feed rate, thereby making it possible to improve machining efficiency.
As shown in FIG. 1, the cutting edges of a conventional spherical cutting tool 10, like a ball end mill, consist of a portion of spherical cutting edge (also called spherical edge) 12 and a body with peripheral cutting edges (also called peripheral edge) 14. The spherical edge 12 serves as the main cutting edge, while the peripheral cutting edge 14 serves as auxiliary cutting edge.
In the case of cylindrical cutting tools, like square end mills, where the main cutting edges are formed on the periphery, the cutting edge has a constant helix angle relative to the tool axis. Thus by providing a proper helix angle to the cutting conditions, the performance of the tool can be markedly improved. If a cutting tool has a constant helix angle, the lengths of the cutting edges are extended, and therefore, the cutting force per unit length is reduced. Further, continuous cutting takes place and impact is minimized during the cutting. Consequently, surface roughness of the cut surface is made fine, precise cutting is possible, and the life expectancy of the tool is extended. That is because the helix angle also functions as the rake angle, and therefore, in the case of a cylindrical cutting tool, 30 degrees of constant helix angle is recommended for cutting steel, while 45 degrees is used for cutting aluminum and its alloys.
The helix angle is closely related to the lead and the tool diameter. As shown in FIG. 2, this relationship can be expressed by a formula tan H =pi D/L, where H is the helix angle, L is the lead, and D is the diameter of the tool. In the peripheral cutting edges of the spherical tool or in the main cutting edges of the cylindrical tool, if the lead is fixed, the helix H has a constant value, or an inverse case is realized. Therefore, the above advantages can be easily realized.
However, in spite of the fact that the performance of the tools can be significantly improved by providing a constant helix of a certain angle, the prior art does not include any such tool in which the spherical cutting edges have a certain constant helix angle relative to the tool axis. Instead, the cutting edges of prior art tools are arranged on the spherical surface in such a manner as to blend well with the peripheral cutting edges, without any coherent relationship or law. Examples of some prior art curves are illustrated in FIGS. 3A and 3B. This is due to the following reason.
Each cross section of the cylinder has the same diameter regardless of axial positions, while in a sphere the diameter D of each cross section is varied along the axial positions X, as illustrated in FIG. 4. Owing to this characteristic, even if the lead is decided, the tool diameter is varied along the tool axis, and therefore, the helix angle H which is based on the formula tan H=Pi D/L has to have a different value at each position. As a result, a constant helix angle cannot be expected.
Meanwhile, if any curve 51, 52, or 53 is located on a sphere connects two points 54 and 56 intersecting the tool axis and the surface of sphere is rotated around the tool axis, the track forms a sphere as shown in FIG. 5. Therefore, if cutting edges are arranged on a sphere regardless of the shape and constancy of the helix angle, and if a rotary cutting is carried out with this tool, then a concave sphere with the same size of the sphere on the tool is formed. Because any helix angle will still produce a tool that forms a properly shaped cut, there is no demand for spherical cutting tools with constant helix. That is, even if the efficiency and performance are poor, the designed shape can be produced, and therefore, the need for the tool of the present invention is not properly recognized. Therefore, conventional spherical cutting tools such as ball end mills are far inferior in their performance to cylindrical cutting tools having a constant helix angle like in the square end mills.
The present invention is intended to overcome the above described disadvantages of the conventional tools by modifying the contour of the cutting edges.
Therefore it is an object of the present invention to provide a spherical cutting tool in which the conventional disadvantages are overcome by arranging the geometrical points of the cutting edges or their tracks.
In achieving the above object, the spherical cutting tool according to the present invention is characterized in that the tangential lines of the respective points of the cutting edges on a spherical surface are formed so as to have a constant angle relative to the axis of the tool, and thus, a machining efficiency is realized in the main cutting edges of the spherical cutting tool like in the main cutting edges of the cylindrical cutting tool.
Further, in achieving the above object, the spherical cutting tool according to the present invention is characterized in that the cutting edges are disposed on a spherical surface, and the helix angle of the cutting edges is constant at any position on the spherical surface. Further, based on the constant helix angle on the spherical surface, the tooth should preferably have a wider land with a smaller relief angle.
Further, the tooth should be made of a material same as that of the tool body.
Further, the tooth made of a super hard alloy metal or a high-speed tool steel may be bonded to the steel body by brazing or by a mechanical means.